linux-kernel - [PATCH 1/2] timers/nohz: Revise a comment about broken iowait counter update race

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Message-Id: <20251129133526.1460119-2-jackzxcui1989@163.com>
Date: Sat, 29 Nov 2025 21:35:25 +0800
From: Xin Zhao <jackzxcui1989@....com>
To: anna-maria@...utronix.de,
	frederic@...nel.org,
	mingo@...nel.org,
	tglx@...utronix.de,
	kuba@...nel.org
Cc: jackzxcui1989@....com,
	linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: [PATCH 1/2] timers/nohz: Revise a comment about broken iowait counter update race

Task migration during wakeup may cause /proc/stat iowait regression,
as mentioned in the commit message of Frederic Weisbecker's previous
patch. The nr_iowait statistic of rq can be decreased by remote CPU
during wakeup, leading to a sudden drop in /proc/stat iowait calculation,
while /proc/stat idle statistic experiences a sudden increase.
Excluding the hotplug scenario when /proc/stat idle statistic may
experiences a sudden decrease which is fixed in a subsequent patch,
/proc/stat idle statistic will never decrease suddenly, as there is no
logic in kernel that allows a remote CPU to increase rq nr_iowait.

Signed-off-by: Xin Zhao <jackzxcui1989@....com>
---
 kernel/time/tick-sched.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 8ddf74e70..4d089b290 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -812,8 +812,8 @@ static u64 get_cpu_sleep_time_us(struct tick_sched *ts, ktime_t *sleeptime,
  * Return the cumulative idle time (since boot) for a given
  * CPU, in microseconds. Note that this is partially broken due to
  * the counter of iowait tasks that can be remotely updated without
- * any synchronization. Therefore it is possible to observe backward
- * values within two consecutive reads.
+ * any synchronization. Therefore it is possible to observe sudden
+ * increases within two consecutive reads.
  *
  * This time is measured via accounting rather than sampling,
  * and is as accurate as ktime_get() is.
-- 
2.34.1